1. Technical Field
The present art relates to an amplitude detecting device in the field of a wireless communication machine and, in particular, an amplitude detecting device, which is used in a transmitting section or a receiving section and can detect a change in the amplitude even when the amplitude of a transmitted or a received signal varies slightly.
2. Description of the Related Art
In recent communication by mobile units, the strength of a transmitted signal, that is, a transmission power is controlled according to the strength of a received signal from a base or relay station for the purpose of effectively using power. The transmission power is also controlled so as to satisfy the transmission power provided for the unit. Therefore, it is necessary to detect the strength of the transmitted and received signal, and Received Signal Strength Indicator (RSSI) has been often used to detect them.
The RSSI has a configuration including multiple serially connected circuit blocks each of which includes an amplifier for amplifying an input signal, and a rectifier for rectifying the input signal or the amplified signal, and an amplitude detector for detecting the current corresponding to the rectified signal. The currents generated by each of the circuit blocks are added, and a high frequency component of the signal is removed through a low-pass filter (LPF). The DC current is converted to a voltage through a resistor. The operation above can provide the voltage according to the amplitude of the input signal, and the magnitude in the range from small amplitude to large amplitude can be obtained as an absolute value of the voltage. This is described in Chun-Pang Wu and Hen-Wai Tsao, “A 110-MHz 84-dB CMOS Programmable Gain Amplifier With Integrated RSSI Function”, IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 40, NO. 6, JUNE 2005.
However, the change in output voltage of RSSI to the change in amplitude of an input signal is constrained. This is because RSSI has to reflect the change in wide-ranging input as the monotonously output voltage, and the designer has to protect the circuit from saturating by the output voltage corresponding to the added current. In other words, the ratio of the change in output voltage to the change in amplitude of an input signal may sometimes not be increased.
In a case where the ratio may not be increased like a conventional device, the changing of output voltage may not be detected with high precision, when the changing of input signal is small. On the other hand, in a case where the output voltage is changed abruptly to the input, that is, in a case where the ratio is too high, the detected voltage may be higher than the upper limit of the circuit, when the amplitude of an input signal is large, so that RSSI can not generate the voltage corresponding to input signal.